1. Field of the Invention
The present invention relates to a broadband wireless communication system, and more particularly to a system and a method for performing a handover in a broadband wireless access communication system employing an orthogonal frequency division multiplexing scheme.
2. Description of the Related Art
In the fourth generation (‘4G’) communication system, which is the next generation communication system, research is being heavily conducted to provide users with services having various qualities of service (‘QoS’) and supporting a transmission speed of about 100 Mbps. The current third generation (‘3G’) communication system supports a transmission speed of about 384 kbps in an outdoor channel environment having a relatively unfavorable channel environment, and supports a maximum transmission speed of 2 Mbps even in an indoor channel environment which is a relatively favorable channel environment. A wireless local area network (‘LAN’) system and a wireless metropolitan area network (‘MAN’) system generally support transmission speeds of 20 to 50 Mbps. Accordingly, in the current 4G communication system, research is being conducted to develop a new type of communication system for ensuring mobility of terminals and a QoS in the wireless LAN and MAN systems supporting relatively high transmission speeds, and supporting a high speed service to be provided by the 4G communication system.
Since the wireless MAN system has a wide service coverage and supports a high transmission speed, it is suitable for supporting a high speed communication service. However, since the wireless MAN system does not in any way take into consideration the mobility of a user, i.e., a subscriber station (‘SS’), the wireless MAN system does not reflect in any way a handover according to high speed movement of the SS. The wireless MAN system is a broadband wireless access (BWA) communication system, which has a wider service coverage and supports a higher transmission speed than the wireless LAN system. The IEEE 802.16a communication system employs an orthogonal frequency division multiplexing (‘OFDM’) scheme and an orthogonal frequency division multiplexing access (‘OFDMA’) scheme in order to enable a physical channel of the wireless MAN system to support a broadband transmission network. That is, the IEEE 802.16a communication system is a broadband wireless access communication system employing an OFDM/OFDMA scheme.
The IEEE 802.16a communication system only considers a state in which an SS is currently motionless (i.e., a state in which the mobility of the SS is not entirely considered), and a single cell structure. However, an IEEE 802.16e communication system is stipulated as a system that takes into consideration the mobility of an SS in the IEEE 802.16a communication system. Accordingly, the IEEE 802.16e communication system must consider the mobility of an SS in a multi-cell environment. In order to support the mobility of the SS in a multi-cell environment, changes in operations of the SS and a base station (BS) are necessarily required. In order to support the mobility of the SS, research into a handover of the SS considering a multi-cell structure has been actively pursued. An SS having the mobility will be referred to as a mobile subscriber station (‘MSS’).
A construction of the IEEE 802.16e communication system will be described with reference to FIG. 1.
FIG. 1 is a diagram illustrating a construction of a conventional IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16e communication system has a multi-cell structure consisting of a first cell 100 and a second cell 150. Further, the IEEE 802.16e communication system includes a base station 110 controlling cell 100, a base station 140 controlling cell 150, and a plurality of MSSs 111, 113, 130, 151, and 153. The transmission/reception of signals between the base stations 110 and 140 and the MSSs 111, 113, 130, 151, and 153 is executed according to the OFDM/OFDMA scheme. From among the MSSs 111, 113, 130, 151 and 153, the MSS 130 is located in a cell boundary area, i.e., handover area, between cell 100 and cell 150. Accordingly, only when a handover for the MSS 130 is supported, is it possible to support the mobility for the MSS 130.
In the IEEE 802.16e communication system, a certain MSS receives pilot channel signals transmitted from a plurality of base stations, and measures the CINRs (Carrier to Interference and Noise Ratios) of the received pilot channel signals. The MSS then selects a base station, which is the base station that has transmitted a pilot channel signal having the highest CINR from among the CINRs measured, as a base station (i.e., as a serving base station) to which the MSS currently belongs. The MSS recognizes a base station, which transmits a pilot channel signal capable of being favorably received by the MSS, from among base stations having transmitted pilot channel signals as a base station to which the MSS belongs.
As a result, the base station to which the MSS currently belongs becomes a serving base station. The MSS having selected the serving base station receives a downlink frame and an uplink frame transmitted from the serving base station. The structure of the downlink frame of the conventional IEEE 802.16e communication system will be descried with reference to FIG. 2.
FIG. 2 is a diagram illustrating the structure of the downlink frame of the conventional IEEE 802.16e communication system.
The downlink frame includes a preamble portion 200, a broadcast control portion 210, and a plurality of time division multiplex (‘TDM’) portions 220 and 230. A synchronization signal (i.e., preamble sequence) used to obtain a mutual synchronization between a base station and an MSS is transmitted through the preamble portion 200. The broadcast control portion 210 includes a downlink MAP (‘DL_MAP’) portion 211 and an uplink MAP (‘UL-MAP’) portion 213. The DL_MAP portion 211 is a portion through which a DL_MAP message is transmitted. Table 1 shows information elements (‘IEs’) contained in the DL_MAP message.
TABLE 1SyntaxSizeNotesDL_MAP_Message_Format( ) { Management Message Type=2 8 bits PHY Synchronization FieldVariableSee Appropriate PHYspecification DCD Count 8 bits Base Station ID48 bits Number of DL_MAP Elements n16 bits Begin PHY specific section {See Applicable PHY section  for (i=1;  i<=n;  i++) {For each DL_MAP element 1 to n   DL_MAP_Information_Element( )VariableSee corresponding PHYspecification   If!(byte boundary) {    Padding Nibble 4 bitsPadding to reach byte boundary   }  } }}
As shown in Table 1, the DL_MAP message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the ‘PHYsical (PHY) Synchronization’ set including a modulation scheme and demodulation scheme information applied to a physical channel in order to obtain a synchronization, the ‘DCD count’ representing a count corresponding to the configuration variation of a downlink channel descript (‘DCD’) message containing a downlink burst profile, the ‘Base Station ID’ representing a base station identifier (BSID), and the ‘Number of DL_MAP Elements n’ representing the number of elements existing after the Base Station ID. The DL_MAP message also contains information about the ranging codes assigned to each ranging, which will be described later.
The UL_MAP portion 213 is a portion through which an UL_MAP message is transmitted. Table 2 shown below illustrates IEs contained in the UL_MAP message.
TABLE 2SyntaxSizeUL_MAP_Message_Format( ) { Management Message Type=3 8 bits  Uplink Channel ID 8 bits UCD Count 8 bits Number of UL_MAP Elements n16 bits Allocation Start Time32 bits Begin PHY specific section {  for (i=1;  i<n;  i+n) {    UL_MAP_Information_Elements {Variable        Connection ID        UIUC         Offset     }  } }}
As shown in Table 2, the UL_MAP message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the ‘Uplink Channel ID’ representing an uplink channel identifier, the ‘UCD count’ representing a count corresponding to the configuration variation of an uplink channel descript (‘UCD’) message containing an uplink burst profile, and the ‘Number of UL_MAP Elements n’ representing the number of elements existing after the UCD count. Herein, the uplink channel identifier is uniquely assigned in a medium access control (‘MAC’) sub-layer.
An uplink interval usage code (‘UIUC’) portion is a portion in which information designating the use of an offset recorded in the offset portion is recorded. For instance, when a value of 2 is recorded in the UIUC portion, it signifies that a starting offset used in the initial ranging is recorded in the offset portion. When a value of 3 is recorded in the UIUC portion, it signifies that a starting offset used in the maintenance ranging or the bandwidth request ranging is recorded in the offset portion. As described above, the offset portion is a portion for recording a starting offset value used in the initial ranging, the bandwidth request ranging, or the maintenance ranging according to the information recorded in the UIUC portion. Further, information about a characteristic of a physical channel to be transmitted in the UIUC portion is recorded in the UCD message.
If an MSS does not successfully perform ranging, the MSS determines a certain backoff value in order to increase the probability of success in the next attempt. The MSS, then, again attempts a ranging process after waiting for the time interval of the backoff value. In this case, the information required for determining the backoff value is also contained the UCD message. The configuration of the UCD message will now be described in more detail with reference to Table 3.
TABLE 3SyntaxSizeNotesUCD-message_Format( ) { Management Message Type=08 bits Uplink Channel ID8 bits Configuration Change Count8 bits Mini-slot size8 bits Ranging Backoff Start8 bits Ranging Backoff End8 bits Request Backoff Start8 bits Request Backoff End8 bits TLV Encoded Information for the overall channelVariable Begin PHY Specific Section {  for(i=1;  i<n;  i+n)     Uplink_Burst_DescriptorVariable  } }}
As shown in Table 3, the UCD message includes a plurality of IEs, that is, the ‘Management Message Type’ representing the type of a transmitted message, the ‘Uplink Channel ID’ representing an uplink channel identifier, the ‘Configuration Change Count’ counted by a base station, the ‘Mini-slot Size’ representing the size of a mini-slot of an uplink physical channel, the ‘Ranging Backoff Start’ representing a start point of a backoff for an initial ranging (i.e., the size of an initial backoff window for the initial ranging), the ‘Ranging Backoff End’ representing an end point of a backoff for an initial ranging (i.e., the size of a final backoff window), the ‘Request Backoff Start’ representing a start point of a backoff for ‘contention data and requests’ (i.e., the size of an initial backoff window), and the ‘Request Backoff End’ representing an end point of a backoff for ‘contention data and requests’ (i.e., the size of a final backoff window). The value of the backoff represents a kind of waiting time value for which an MSS must wait for the next ranging when failure occurs in rangings. Further, a base station must transmit the backoff value, which is information relating to a time period for which the MSS must wait for the next ranging, to the MSS when the MSS fails in a ranging. For instance, when a value by the Ranging Backoff Start and the Ranging Backoff End is set to 10, the MSS passes chances in which the MSS can perform rangings of 210 times (i.e., 1024 times) according to a truncated binary exponential backoff algorithm and then must perform the next ranging.
In addition, the TDM portions 220 and 230 are portions corresponding to time slots assigned to each MSS by a time division multiplex (‘TDM’)/time division multiple access (‘TDMA’) scheme. The base station transmits broadcast information to MSSs managed by the base station through the DL_MAP portion 211 of the downlink frame by means of a preset center carrier. When the MSSs are powered on, each MSS monitors all of the frequency bands set in each MSS itself in advance and detects a pilot channel signal having the highest pilot CINR. The MSS determines a base station having transmitted the pilot channel signal having the highest CINR to be a base station to which the MSS currently belongs. The MSS confirms the DL_MAP portion 211 and the UL_MAP portion 213 of the downlink frame transmitted by the base station, and confirms control information for controlling an uplink and a downlink of the MSS and information representing the actual position of data transmission/reception.
FIG. 3 is a diagram illustrating the structure of the uplink frame of the conventional IEEE 802.16e communication system.
Before describing FIG. 3, rangings used in the IEEE 802.16e communication system, an initial ranging, a maintenance ranging (i.e., periodic ranging), and a bandwidth request ranging will be described in detail.
First, the initial ranging will be described.
The initial ranging is a ranging which is performed when a base station requests the initial ranging in order to obtain synchronization with an MSS. Further, the initial ranging is a ranging which is performed in order to match an exact time offset between the MSS and the base station and adjust the transmit power. That is, the MSS is powered on, receives a DL_MAP message, an UL_MAP message and a UCD message and obtains synchronization with the base station. Then, the MSS performs the initial ranging to adjust the time offset and the transmit power with the base station. Since the IEEE 802.16e communication system employs an OFDM/OFDMA scheme, the ranging procedure requires ranging sub-channels and ranging codes. A base station assigns usable raging codes according to the object of each raging, that is, each kind of raging. This will now be described in detail.
The raging code is generated by segmenting a pseudo-random noise (‘PN’) sequence having a predetermined length (e.g., length of 215−1 bits) by a predetermined unit. Generally, two ranging sub-channels having a length of 53 bits constitute one ranging channel. The raging code is constructed by segmenting a PN code through the ranging channel having a length of 106 bits. The 48 raging codes (i.e., RC#1 to RC#48) (at a maximum of 48 ranging codes per MSS) constructed in this way may be assigned to an MSS, and two raging codes (at a minimum per each MSS) are applied to the three types of rangings, that is, the initial ranging, the periodic ranging and the bandwidth request ranging, as a default value. In this way, different raging codes are assigned to each ranging. For instance, N raging codes are assigned for the initial ranging (N RCs for initial ranging), M raging codes are assigned for the periodic ranging (M RCs for periodic ranging), and L raging codes are assigned for the bandwidth request ranging (L RCs for BW-request ranging). The raging codes assigned in this way are transmitted to the MSSs through the DL_MAP message as described above, and the MSSs perform the ranging procedure by using the raging codes contained in the DL_MAP message according to the objects of the raging code.
Second, the periodic ranging will now be described.
The periodic ranging is a ranging periodically performed when the MSS having adjusted the time offset and the transmit power with the base station through the initial ranging adjusts a channel status, etc., with the base station. The MSS performs the periodic ranging by means of the ranging codes assigned for the periodic ranging.
Third, the bandwidth request ranging will now be described.
The bandwidth request ranging is a ranging performed when the MSS having adjusted the time offset and the transmit power with the base station through the initial ranging requests a bandwidth assignment in order to actually perform a communication with the base station. The bandwidth request ranging may be performed using one selected from among a Grants scheme, a scheme of ‘Contention-based Focused bandwidth requests for Wireless MAN-OFDM’, and a scheme of ‘Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA’. Each of the Grants scheme, the ‘Contention-based Focused bandwidth requests for Wireless MAN-OFDM’scheme, and the ‘Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA’scheme will be described in detail.
(1) Grants Scheme
The Grants scheme is a scheme for requesting a bandwidth assignment when a communication system to which an MSS currently belongs is a communication system using a single carrier. In this case, the MSS performs the bandwidth request ranging using not its own connection identifier (‘CID’) but a default CID. When the bandwidth request ranging fails, the MSS again attempts the bandwidth request ranging after a time interval of a backoff value or abandons a received service data unit (SDU), according to the latest information received from the base station and a request condition of the base station. In this case, the MSS already recognizes the backoff value through a UCD message.
(2) ‘Contention-Based Focused Bandwidth Requests for Wireless MAN-OFDM’ scheme
The ‘Contention-based Focused bandwidth requests for Wireless MAN-OFDM’ scheme is a scheme for requesting a bandwidth assignment when a communication system to which an MSS currently belongs is a communication system using the OFDM scheme. The ‘Contention-based Focused bandwidth requests for Wireless MAN-OFDM’ scheme is classified into two schemes. The first scheme is a scheme of performing a bandwidth request ranging in such a manner that an MSS uses a default CID, as described in the description for the Grants scheme, and simultaneously transmits a focused contention transmission message. The second scheme is a scheme of performing a bandwidth request ranging by transmitting a broadcast CID together with an OFDM Focused Contention ID, not using the default CID. When the broadcast CID together with the OFDM Focused Contention ID is transmitted, the base station determines a specific contention channel and a transmission probability for the relevant MSS.
(3)‘Contention-Based CDMA Bandwidth Requests for Wireless MAN-OFDMA’ scheme
The ‘Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA’ scheme is a scheme for requesting a bandwidth assignment when a communication system to which an MSS currently belongs is a communication system using the OFDMA scheme. The ‘Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA’ scheme is also classified into two schemes. The first scheme is a scheme of performing a bandwidth request ranging CID as described in the description about the Grants scheme. The second scheme is a scheme of performing a bandwidth request ranging using a mechanism based on CDMA (Code Division Multiple Access) scheme, that is, using a CDMA based mechanism. In the CDMA based mechanism, since the communication system uses a plurality of tones (i.e., a plurality of sub-channels) made-up with OFDM symbols, the base station applies a mechanism such as the CDMA scheme to each of the sub-channels when an MSS performs a bandwidth request ranging. When the base station successfully receives the bandwidth request ranging, the base station assigns a frequency band through a MAC protocol data unit (PDU) to the MSS having performed the bandwidth request ranging. Meanwhile, in a case of using an REQ (REQuest) Region-Focused scheme, the possibility of a collision increases when a plurality of MSSs attempt bandwidth request rangings using the same contention code through the same sub-channel.
Referring to FIG. 3, the uplink frame includes an ‘Initial Maintenance Opportunities’ portion 300 for the initial ranging and the maintenance ranging (i.e., periodic ranging), a ‘Request Contention Opportunities’ portion 310 for the bandwidth request ranging, and an ‘MSS scheduled data’ portion 320 containing the uplink data of the MSSs. The Initial Maintenance Opportunities portion 300 includes a plurality of access burst intervals actually containing an initial ranging and a periodic ranging, and a collision interval in which collision between access burst intervals occurs. The Request Contention Opportunities portion 310 includes a plurality of bandwidth request intervals contains a bandwidth request ranging and a collision interval in which collision between bandwidth request intervals occurs. Further, the MSS scheduled data portion 320 includes a plurality of MSS scheduled data parts (i.e., MSS 1 scheduled data part to MSS N scheduled data part) and MSS transition gaps each of which is present between the adjacent MSS scheduled data parts.
FIG. 4 is a flow diagram illustrating the first ranging procedure between a base station and an MSS in the conventional IEEE 802.16e communication system. The MSS 400 monitors all of the frequency bands in the MSS 400 in advance and detects a pilot channel signal having the highest CINR. Then, the MSS 400 determines a serving base station 420 having transmitted the pilot channel signal having the highest CINR to be the serving base station 420 (i.e., a serving base station) to which the MSS 400 currently belongs. Then, the MSS 400 receives the preamble of the downlink frame transmitted from the serving base station 420 and obtains a system synchronization with the serving base station 420.
When the system synchronization is obtained between the MSS 400 and the serving base station 420 as described above, the serving base station 420 transmits a DL_MAP message and an UL_MAP message to the MSS 400 in steps 411 and 413, respectively. Herein, as described in Table 1, the DL_MAP message functions to inform the MSS 400 of information required when the MSS 400 obtains a synchronization with the serving base station 420 in a downlink, and information about the structure of a physical channel capable of receiving messages transmitted to the MSS 400 in the downlink. Further, as described in Table 2, the UL_MAP message functions to inform the MSS 400 of information about the scheduling period of an MSS and the structure of a physical channel in an uplink. Meanwhile, the DL_MAP message is periodically broadcast from a base station to all of the MSSs. Herein, when a certain MSS can continuously receive the DL_MAP message, it can be expressed that the MSS has synchronized with the base station.
The MSSs having received the DL_MAP message can receive all of the messages transmitted through a downlink. Further, as described in Table 3, when an MSS fails in an access, the base station transmits the UCD message containing information notifying the MSS of an usable backoff value.
When the MSS 400 having been synchronized with the serving base station 420 performs the ranging, the MSS 400 transmits a ranging request (‘RNG_REQ’) message to the serving base station 420 in step 415. Then, in step 417, the serving base station 420 having received the RNG_REQ message transmits to the MSS 400 a ranging response (‘RNG_RSP’) message, which contains information for compensating for frequency, time, and transmit power for the ranging.
Table 4, illustrated below, shows the configuration of the RNG_REQ message.
TABLE 4SyntaxSizeNotesRNG_REQ_Message_Format( ) { Management Message Type=48 bits Downlink Channel ID8 bits Pending Until Complete8 bits TLV Encoded InformationVariableTLV specific}
In Table 4, the ‘Downlink Channel ID’ represents a downlink channel identifier contained in the RNG_REQ message received in the MSS 400 through the UCD. The ‘Pending Until Complete’ represents a priority of a transmitted ranging response. That is, when the Pending Until Complete has a value of 0, a previous ranging response has a high priority. In contrast, when the Pending Until Complete has values other than 0, a currently transmitted ranging response has a high priority.
Table 5, illustrated below, shows the configuration of the RNB_RSP message in response to the RNG_REQ message shown in Table 4.
TABLE 5SyntaxSizeNotesRNG_RSP_Message_Format( ) { Management Message Type=58 bits Uplink Channel ID8 bits TLV Encoded InformationVariableTLV specific}
In Table 5, the ‘Uplink Channel ID’ represents an uplink channel identifier contained in the RNG_REQ message.
In a case of using the OFDMA scheme in the IEEE 802.16e, in order to more efficiently perform the first ranging procedure as described above, a scheme of establishing a dedicated section for the ranging and transmitting ranging codes through the dedicated section may be used, instead of using the RNG_REQ message. A ranging procedure between a base station and an MSS when the scheme of transmitting ranging codes only through the dedicated section will now be described with reference to FIG. 5.
FIG. 5 is a flow diagram illustrating the second ranging procedure between a base station and an MSS in the conventional IEEE 802.16e communication system.
Referring to FIG. 5, the second ranging procedure between the base station and the MSS basically includes the same steps as those of the first ranging procedure described with reference to FIG. 4. However, according to the second ranging procedure, the MSS 500 transmits a ranging code to the serving base station 520 before transmitting the RNG_REQ message in step 515. Then, the serving base station 520 receives the ranging code and then transmits the RNG_RSP message to the MSS 500 in step 517. In step 519, the MSS 500 having received the RNG_RSP message transmits the RNG_REQ message to the serving base station 520 through a contention-free band assigned by the serving base station 520.
Meanwhile, the serving base station inserts response information to the received ranging code into the RNG_RSP message. In this case, information newly-contained in the RNG_RSP message is as follows.                a. Ranging Code: Received ranging CDMA code.        b. Ranging Symbol: OFDM symbol of the received ranging CDMA code.        c. Ranging sub-channel: Sub-channel of the received ranging CDMA code.        d. Ranging frame number: Frame number of the received ranging CDMA code.        